WO2005108768A1

WO2005108768A1 - Acoustic power transmitting unit for thermoacoustic systems

Info

Publication number: WO2005108768A1
Application number: PCT/FR2005/050299
Authority: WO
Inventors: Emmanuel Bretagne; Maurice-Xavier Francois
Original assignee: Universite Pierre Et Marie Curie; Centre National De La Recherche Scientifique
Priority date: 2004-05-04
Filing date: 2005-05-03
Publication date: 2005-11-17
Also published as: CN1981127A; EP1745206B1; ATE478250T1; CA2565573C; DK1745206T3; US20080276625A1; EP1745206A1; FR2869945A1; FR2869945B1; US8640467B2; CA2565573A1; DE602005023004D1

Abstract

The invention relates to an acoustic power transmitting unit for thermoacoustic systems comprising at least on stage ptovided with at least two thermoacoustic units (3, 4, 12-16) each of which comprises a regenerator or a stack and two heat exchangers, an acoustic resonator comprising a tube and containing a fluid and in which a magnetic field consisting of high impedance and low-impedance areas is arranged, wherein certain thermoacoustic units (3, 4, 12-16) are placed in high dimensionless impedance areas. According to said invention, each high dimensionless impedance area also comprises a thermoacoustic unit, wherein two successive thermoacoustic units (3, 4, 12-16) are always separated by low dimensionless impedance. The resonator comprises a reduced diameter section (5, 21-23) between each couple of successive thermoacoustic units (3, 4, 12-16) and each cross-sectional narrowing (5, 21-23) is associated with at least one by-pass (6,7) which comprises a deviation cavity (10, 11) and makes it possible to deviate the major part of a volume flow rate.

Description

Acoustic power transmission unit for thermoacoustic systems

This invention relates to thermal machines, motors and refrigerators employing a process for converting thermoacoustic energy. In particular, it relates to thermoacoustic machines of all types which include wave generators and thermoacoustic refrigerators, but also the family of Stirling and Ericsson machines and the family of pulsed gas tubes. Any thermal machine requires at least the presence of two heat sources at different temperatures, a mechanical work transmission system and an energy conversion agent describing a thermodynamic cycle. In a thermoacoustic machine mechanical work takes the form of acoustic work, more commonly expressed per unit of time in terms of acoustic workflow or acoustic power and corresponding to the time average of the product of acoustic pressure by volume flow. acoustic. The concept of thermodynamic cycle and therefore of energy conversion is the basis of the operation of any thermal machine. In heat engines a quantity of heat is converted into acoustic work and in refrigerators a quantity of work is consumed to transfer heat from a medium at so-called "low" temperature to another medium at higher temperature. The power of thermal machines is directly linked to the "opening" of the thermodynamic cycle, ie the area formed by this cycle. In most non-acoustic machines, such as a domestic refrigerator operating according to the Rankine thermodynamic cycle for example, the conversion agent which describes the thermodynamic cycle is a fluid. This fluid is called "refrigerant", and is circulated in a closed circuit where it vaporizes and condenses. In thermoacoustic machines, the conversion agent is generally a gas, typically helium, and the thermodynamic cycle is implemented by an acoustic wave on a smaller scale corresponding to that of the displacement of a particle of fluid which oscillates. It is the cooperation of all local thermodynamic cycles, cooperation naturally synchronized by the wave itself, which allows conversion of energy on the global scale of the engine (also called wave generator) or of the thermoacoustic refrigerator. In a thermoacoustic system the thermodynamic cycle takes place only in the contact zone, or acoustic thermal boundary layer, between the fluid subjected to compression-expansion phases by the acoustic wave and a solid medium which produces the necessary heat sources at the "opening" of the thermodynamic cycle. This boundary layer fluid / solid interaction which results in heat exchanges between the fluid and the solid results from temperature oscillations which accompany any acoustic propagation. This fluid / solid interaction involves the dilatability of the fluid. In a thermoacoustic system, the local thermodynamic cycles accomplished may, depending on the type of sound field, be more akin to Brayton cycles or rather to Ericsson and Stirling cycles. We obtain a first type of operation called, 'Brayton', when the acoustic wave is similar to a rather stationary wave, that is to say having a phase shift between acoustic pressure and particle displacement close to

180 °, and a second type of operation known as 'Ericsson or Stirling' when the wave is rather progressive, that is to say has a phase shift between acoustic pressure and particle displacement close to 90 °. The realization of the local thermodynamic cycle requires that thermodynamic transformations follow one another in a coordinated manner over time. Thus the heat inputs are such that the fluid of a thermoacoustic wave generator locally performs a thermal expansion (expansion) when its pressure is maximum and a thermal contraction when its pressure is minimum. Thermal extension occurs when the fluid receives heat and vice versa. The synchronization of thermodynamic transformations which translates an 'arrangement' between the phases of displacement, compression-expansion and extension-contraction of the fluid is achieved by the acoustic wave. The solid medium is presented as a more or less dense and relatively uniform matrix allowing good propagation of the acoustic waves insofar as the typical dimensions are much less than the wavelength corresponding to the acoustic field. This solid medium consists of a set of pores or channels, placed in parallel, allowing the passage of a fluid from one end to the other of the matrix. These channels can have very varied shapes, and are not necessarily identical. This active solid matrix, in which the fluid oscillates, necessarily has a different characteristic of appearance δ _κ / R _h to allow the realization of the two types of operation described above. δ _κ designates the thickness of thermal boundary layer and is defined by δ _κ = 2— where K is the thermal diffusivity of the fluid taken at the average temperature V ω of this same fluid and ω the pulsation of the acoustic wave. R _h denotes the mean hydraulic radius of the solid matrix taken in the sense of porous media. Thus in the first type of operation known as of 'Brayton', δ _κ is of the order of R _h and one then commonly calls the solid matrix, a “stack”. In the second type of operation called 'Ericsson or Stirling', δ _κ is much greater than R _h and the solid matrix is then called a "regenerator", with reference to Stirling regeneration machines. While in a regenerator, a good thermal contact is established between the solid elements and the gas, on the contrary this contact is not good in the stacks. In the case of a regenerator, the phase difference between the acoustic pressure and the acoustic speed is close to zero or has an area where the phase difference is zero. On the contrary in the case of the stack, this phase shift is always significant and close to 90 °. Both the regenerator and the stack are organs subjected to a stationary temperature distribution, despite the oscillating movement of the fluid, because they are placed between two "sources" of heat. A spatial distribution of heat sources is therefore established with temperatures intermediate to those of the two external heat sources. Proper operation, both of a stack and of a regenerator, requires that they are each placed between two heat exchangers maintained at constant and different temperatures in order to constitute a thermal machine. The terms “stack unit” or “regenerator unit” are then used to designate a stack or a regenerator placed between two heat exchangers. The temperature distribution both in a generator and in a stack is imposed in the case of an engine operation, by the supply of heat to one of the heat exchangers of the regenerator unit or of the stack unit. The heat can be obtained from electrical, nuclear or solar energy, by combustion, or by recovery of any thermal rejection at the appropriate temperature. It is the local temperature gradients, consecutive to the temperature distribution, which are responsible for the conversion of thermal energy into acoustic energy and thus the generation of acoustic waves of high acoustic power. In the case of refrigerator operation, the temperature distribution in the regenerator is generated by the acoustic wave. Stack units can be used in engine operation to generate thermoacoustic power in a thermoacoustic machine, thus producing the same effect as an acoustomechanical engine but with the advantage of having no moving mechanical parts. Still in engine operation, the regenerator units can be used to amplify the flow of acoustic power generated by the engines or by the stack units in an acoustic resonator. Ideally, the amplification rate of the acoustic power in a regenerator is equal to the ratio of the temperature of the heat exchanger where the heat is supplied to that where the unconverted heat is extracted, the temperatures being expressed in Kelvin . In a regenerator the amplification of the acoustic power flow takes place in the direction corresponding to positive temperature gradients. In refrigerator operation, the stack and regenerator units are used interchangeably to allow heat to be extracted from a medium to be cooled. This heat is transferred to a higher temperature heat exchanger to be evacuated there. The highest temperature can be chosen variably, which has an advantage over many refrigeration technologies such as condensation-vaporization refrigeration for example. It is thus not necessarily close to 293K and may for example be less than 200K for cryogenic applications or greater than 500K for applications in a high temperature environment. The choice of a refrigeration unit in the form of a stack or regenerator unit directly influences the unit's performance coefficient, also called energy conversion coefficient, which is defined as the ratio of the amount of heat extracted to the amount of acoustic work consumed, and the temperature differential between the lowest temperature heat exchanger and the highest temperature heat exchanger. Thus, in accordance with the theoretical yields of the Brayton and Ericsson (or Stirling) cycles, a stack unit does not generally make it possible to obtain a performance coefficient as high as that of a regenerator unit. In addition, a regenerator unit is generally better suited to large temperature differentials than a stack unit. By extension, one also indicates “Regenerator unit”, or “Extended regenerator unit”, a regenerator associated with its two exchangers to which a section of tube is joined and a third heat exchanger. The tube section constitutes a volume of buffer gas making it possible to thermally isolate the hottest exchanger in the case of an acoustic power amplification unit or the coldest in the case of a refrigeration unit. The third exchanger placed at one end participates in controlling the temperature distribution in the tube section. In this particular embodiment and for an application as a refrigeration unit, the regenerator unit is then called a "pulsed gas tube unit". For reasons of stability with regard to the effects of natural convection induced by gravity, the regenerator unit is preferably placed vertically, the exchanger at the highest temperature among the second and third exchangers being placed at altitude. the highest. A thermoacoustic machine thus consists of active thermoacoustic units placed in an acoustic resonator. The resonator has inter alia a role of waveguide. It can be used at its resonant frequency or not. For example in the case of an acoustic energy source consisting of a loudspeaker, it is possible to preferably choose an operating frequency different from the resonance frequency. In the case where the acoustic machine comprises an acoustic wave generator, the geometry of the resonator closely conditions the operating frequency ffbncturement d ^β the device. In a thermoacoustic machine, the impedance Z is defined as being the ratio between the acoustic pressure Pi and the acoustic speed ui. Each of these two parameters Pi and Ui can be measured locally, we can thus access this impedance Z at each point. The index 1 of each parameter specifies that it is an acoustic quantity, infinitely small of the first order.

The dimensionless impedance is the ratio | Z | / pc where is p the density of the fluid contained in the resonator and c is the speed of sound in this same fluid and | Z | the module of Z. It is known that the thermoacoustic units function correctly only in areas where the amplitude of the movements of the fluid particles is reasonably low and where the amplitude of the acoustic pressure is large. This amounts to placing the thermoacoustic units in a zone of high dimensionless impedance.

An objective of this invention is to allow an improvement in the overall performance of a thermoacoustic machine on the thermodynamic level. In particular, this invention proves to be advantageous for the production of a thermoacoustic machine associating one or more units of pulsed gas tubes with a thermoacoustic wave generator composed of stack and regenerator units.

In a thermoacoustic machine comprising more than one thermoacoustic unit, the transmission of acoustic power between two units of stack, regenerator or pulsed gas tube must, of course, be maximum in order to maintain a high energy efficiency for the machine.

Thus, two possible positions are known for placing two thermoacoustic units in an acoustic resonator. These thermoacoustic units can be placed:

- Either consecutively and as closely as possible, which necessarily leads to an almost integral transmission of acoustic power between the two units.

This first arrangement amounts, for example, to placing the units in cascade in the same zone of high dimensionless impedance (see Gregory W. Swift et al. US-6,658,862).

- Either to zones of distinct high dimensionless impedance, each of these zones being separated by a zone of low dimensionless impedance. This second arrangement corresponds, for example, conventionally to the placement of a tube of length close to λ / 2 acoustically between the two units, the Q wavelength λ being such that λ = - where the operation is the frequency / operation of operation of the thermoacoustic machine. However, this second arrangement inevitably leads to greater losses of acoustic power between the two units. These losses are essentially linked to the formation of acoustic turbulence in the zone of low dimensionless impedance which is also generally a zone of high acoustic velocities. The first arrangement therefore seems favorable. However, given the physical size of the thermoacoustic units, optimal operation of each of them cannot be satisfied perfectly in the same area of high dimensionless impedance from more than 3 thermoacoustic units. It is then necessary to use a device for extending the same zone of high dimensionless impedance (Swift et al., US-6,658,862). However, this extension device is inevitably a consumer of acoustic power.

In addition, this first arrangement has few independent adjustment parameters. As a result, the malfunction of a single element of the cascade can be very detrimental to the operation of the assembly.

Of course, the necessary coordination of the thermoacoustic units in the same area of high dimensionless impedance and therefore their adjustment, becomes more and more complex when the number of thermoacoustic units increases. Furthermore, an additional brake on the accumulation of thermoacoustic units in the same zone of high dimensionless impedance is the difficulty of guaranteeing the stability of such a system during operation under variable conditions (for example, in a zone geographic subject to strong temperature differentials between day and night). An object of the present invention is therefore to propose a simple device in its design and in its operating mode allowing a significant transmission of acoustic power between each stack or regenerator unit, or of pulsed gas tube while limiting energy losses. by viscous dissipation mechanisms or by allowing to group in a space reduces several consecutive units without deteriorating their individual performance.

Thus according to the invention, it has been found that it is possible to place each thermoacoustic unit in zones of high dimensionless impedance and to place several of them, in zones of distinct high dimensionless impedance, each of these zones being separated by an area of low dimensionless impedance.

Another object of the invention is to allow the establishment of acoustic parameters in accordance with an optimized operation of each thermoacoustic unit, this essentially independently of the operation of the adjacent thermoacoustic units. This possibility of adjustment and control introduced by the invention is particularly advantageous when the units are grouped. The invention also advantageously makes it possible to reduce the dimensions of such a machine and therefore its size. To this end, the invention relates to a power transmission unit for thermoacoustic systems comprising at least one stage, comprising: - at least two thermoacoustic units each comprising a regenerator or a stack and two heat exchangers, - an acoustic resonator comprising a tube and containing a fluid and in which an acoustic field having zones of high dimensionless impedance and zones of low dimensionless impedance is established, - certain thermoacoustic units being placed in zones of high dimensionless impedance. According to the invention: - each zone of high dimensionless impedance comprises at most one thermoacoustic unit, - two successive thermoacoustic units always being separated by a zone of low dimensionless impedance, - the resonator comprises a section of reduced diameter between each of the pairs of successive thermoacoustic units, and each section narrowing is associated with at least one bypass comprising a cavity, said bypass making it possible to divert at least part of the volume flow rate of the tube. The term "shrinking" is intended to mean an area in which the diameter is reduced relative to the largest tube diameter of the area of high dimensionless impedance. In different embodiments, the present invention also relates to the following characteristics which must be considered in isolation or according to all technically possible combinations: - each section narrowing is associated with two branches, placed respectively at each end of the narrowing; - the narrowing of the section is continuous; By "continuous" is meant progressive variations without jumping as opposed to a "discontinuous" variation illustrated by a step. - the narrowing of the section takes the form of a cone; - the narrowing of the section is discontinuous; - the narrowing of the section takes the form of a step; - Each branch includes a conduit connecting the cavity to the tube; - Each branch additionally includes thermal regulation means making it possible to control the flow rate in the branch; - resistive systems are associated with at least one of the conduits; - It comprises at least one acoustically active element allowing the adaptation of the operating conditions of the thermoacoustic units; - the acoustically active element is a stack unit placed in the derived cavity; - the acoustically active element is a loudspeaker placed in the derived cavity. In different possible embodiments, the invention will be described in more detail with reference to the accompanying drawings in which: - Figure 1 is a schematic representation of a power transmission unit for thermoacoustic systems, according to a first embodiment of the invention; - Figure 2 is a schematic representation of a power transmission and amplification unit for thermoacoustic systems, according to a second embodiment of the invention; - Figure 3 is a schematic representation of a power transmission unit for thermoacoustic systems, according to a third embodiment of the invention; - Figure 4 is a schematic representation of a conduit leading to a derived cavity according to a first embodiment; - Figure 5 is a schematic representation of a conduit leading to a derived cavity according to a second embodiment; - Figure 6 is a schematic representation of a conduit leading to a derived cavity according to a third embodiment; - Figure 7 is a schematic representation of a conduit leading to a derived cavity with a temperature control device according to a fourth embodiment; - Figure 8 is a schematic representation of a conduit leading to a derived cavity, said cavity comprising an acoustically active element according to a fifth embodiment; - Figure 9 is a sectional view of a resonator having multiple leads according to a particular embodiment; - Figure 10 is a schematic representation of a section of tube of reduced diameter with a temperature control device according to an embodiment of the invention; - Figure 11 is a schematic representation of the evolution of the volume flow and the acoustic pressure in the first section of tube of reduced diameter of the transmission unit of Figure 2; - Figure 12 is a schematic representation of the evolution of the volume flow and the acoustic pressure in the second section of tube of reduced diameter of the transmission unit of Figure 2 (Figure 12A) and Figure 12B is a representation schematic of the evolution of the amplitude and the phase of the volume flow and the sound pressure in the second section of tube of reduced diameter of the transmission unit of FIG. 2. Conventionally, the transmission unit power for thermoacoustic systems is an integral part of an acoustic resonator comprising a main tube of any geometry and generally of uniform diameter D. This resonator, in combination with the other elements of the device, defines the frequency of the system and the wavelength corresponding. The main tube according to the invention comprises a first 1 and a second 2 element which are connected by a tube section 5 of reduced diameter d. The ends of the first and second elements 1, 2, connected by said section of tube 5 of reduced diameter, each have a derived cavity or branch 6, 7. Each branch 6, 7 comprises a cavity 8, 9 representing a closed volume connected to a conduit 10, 11, acting on the acoustic characteristics, and in particular on the acoustic volume flow of the main tube (Figure 1). Thermoacoustic cells or units 3, 4 are arranged in the resonator, in zones of high dimensionless impedance, two zones of successive high dimensionless impedance being separated by a zone of low impedance. It is known that the branches 6, 7 make it possible to modify the acoustic parameters and in particular the volume flow at the inlet (or at the outlet) of the section of tube of reduced diameter 5. The invention therefore makes it possible to obtain a transmission optimal sound power between each thermoacoustic unit 3, 4 while maintaining a reduced space requirement of the system. If the value of the flow is very high and the conditions set out above are difficult to satisfy, it is possible to put several leads 6, 7 in parallel to distribute the initial flow (Figure 9). Furthermore, the reduced diameter section 5 can consist of a succession of reductions and increases in diameter. The evolution of the flow in the reduced diameter section can be controlled by acting on the local temperature gradient (Figure 10). It is known that regenerator units have a better energy conversion efficiency than stack units and it is therefore recommended to use regenerator units as much as possible to compose a thermoacoustic machine. The regenerator units however require the injection of acoustic power at their end at 'room' temperature, i.e. at the end from which heat is discharged to the outside of the machine , and cannot be used exclusively in the composition of a thermoacoustic machine with the exception of any source of acoustic power such as a stack unit for example. In the present invention, a preferred embodiment is to combine units in cascade to form a machine and thus provide a large amplification of a small power initially created by a small stack unit or a mechanical acoustic source. The low stack efficiency compared to regenerators thus takes a negligible part in the efficiency total, especially since the amount of power dissipated in the transmission between units remains low. Figure 2 shows such a power transmission and amplification unit for thermoacoustic systems in a second embodiment of the invention. The resonator comprises a stack unit 12 making it possible to produce acoustic power, which will be amplified by regenerator units 13-14 placed in cascade and used by the "pulsed gas tube" units 15-16. These thermoacoustic units 12-16 are each arranged in a zone of high dimensionless impedance in the resonator and are separated from the adjacent unit by a zone of low dimensionless impedance. The resonator therefore comprises a set of 4 main tube elements 17-20 of diameter Dj where j = 1 to 4, which are interconnected by sections or tubes of reduced diameter 21 -23 which may have different lengths. In the diameter Dj where j = 1 to 4 of each main tube, the passing section for the acoustic wave is retained. In fact, the diameter of the resonator can be larger to contain thermal insulation (ceramic fiber) and the actually passing diameter can correspond to the inside diameter of a coaxial tube, itself thin to limit the effects of conduction. thermal. These reduced diameter sections 21 -23 make it possible to optimally transmit the acoustic power through zones of low dimensionless impedance, when at least part of the acoustic volume flow in the main tube element 17-20 has previously been " diverted "into a cavity placed in bypass 24-29. A cavity placed in bypass 24-29 is thus visible near each section change area. In a first embodiment of a pipe element 30 comprising a section of tube of reduced diameter 21 and two branches 24, 25 this has a length equivalent to the acoustic plane to λ / 2, where λ denotes the length of preferred acoustic wave. By "pipe element of length equivalent to the acoustic plane at λ / 2", it is meant that the resonator element is between two zones of high dimensionless impedance and incorporates a section of zero impedance for the preferred acoustic wave. The resonator comprises a first 17 and a second 18 elements connected at one of their ends by a first section of tube 21 of reduced diameter d (Fig. 2). In order to avoid the creation of acoustic power losses by acoustic turbulence in the zone of low dimensionless impedance, which is also generally a zone of high acoustic velocities, the ends of the first 17 and second 18 elements each include a derived cavity 24, 25 comprising a duct 31, 32. Thus by deriving at least part of the volume flow present in the main tube 17, 18 in the derived cavity 24, 25, the device makes it possible to maintain a Reynolds number Re much lower than the critical Reynolds number R _eC ritique beyond which the acoustic turbulence phenomenon appears. This makes it possible at the same time to decrease the dissipation of linear energy, to preserve in the system a laminar acoustic behavior, as well as to privilege a linear modeling. It is known that the effects of turbulence in a resonant tube can cause very large losses, up to 90% of all losses over a length generally equivalent to λ / 2 acoustically. It is moreover known that the acoustic Reynolds number is defined as Re = ^ L_ where d is the diameter of the tube, of great length, v the kinematic viscosity Av of the fluid and A the area of a section of tube. The Reynolds critical acoustics, Re _C ritique _> typically has a value between 10 ⁵ and 10 ⁶ [SM Hino et al. ; Journal of Fluid Mechanics 75 (1976) 193-207]. Reducing the diameter has a detrimental effect on the dissipation by acoustic turbulence except in the sense of the invention for which the volume flow rate U, is reduced at the inlet of the tube. Figure 11 shows a typical variation of the volume flow in the reduced tube 21 and the effect of the derived cavities 24, 25 on the reduction of the flow in the tube. The first curve 33 (in solid line) shows the evolution of the volume flow rate and the second curve 34 (in solid line and circles) shows the evolution of the acoustic pressure in the section of reduced diameter tube 21 of the unit of transmission 30 of FIG. 2. Of course, the reduction in flow rate in the tube will be adapted to the reduction in diameter which makes it possible to reduce the developed length of the device. A second possible embodiment of the pipe element 35 comprising a section of reduced diameter and two branches is shown in FIG. 2, by means of a second tube 22 of reduced diameter d ₂ connecting the other end of the second element 18 to a third element 19 of the main tube. The length equivalent to the acoustic plane of this element of conduct is much less than λ / 4, for example it is typically 15% of λ / 4. By “pipe element of equivalent length much less than λ / 4 acoustically”, it is understood in the context of the invention that the resonator element is between two zones of high impedance and incorporates sections of low impedance but never zero for the preferred acoustic wave. Each of the ends of the second 18 and third 19 main tube elements, are connected via a conduit 36, 37 to a corresponding cavity placed in bypass 38, 39. These cavities 38, 39 and conduits 36, 37 are different because it is thus possible to independently adjust the operating conditions (ie the amplitude and the phase between acoustic pressure and speed) of each regeneration unit 13-16 to recreate operating conditions at the input of each of these units that are optimal. Advantageously, this reduced diameter tube 22 makes it possible to create a zone of low dimensionless impedance over a short length of tube, which thus makes it possible to make the power transmission unit compact. The other end of the third main tube element 19 is connected via a third section 23 of reduced diameter tube d ₃ to one end of a fourth tube element 20. This third section 23 of reduced diameter tube d ₃ and the associated branches 28, 29 form a pipe element of length equivalent to λ / 2 in the acoustic plane. The fourth tube element 20 which completes the main tube is the refrigerator part of the thermoacoustic system. This consists of two pulsed gas tubes with orifice-inertia placed in parallel [Bretagne et al. ; "Investigations of acoustics and heat transfer characteristics of thermoacoustic driven puise tube refrigerators", In proceeding of CEC-ICMC03 - Anchorage]. The paralleling is obtained by the separation of the main tube 20 at its other end into two elements of secondary tube of reduced section. In order to be able to arrange all of the thermoacoustic units extended in the preferred vertical direction with respect to gravity, the tubes are bent at 180 °. In an acoustic resonator, the preferred acoustic wave can either be imposed when using a non-thermal acoustic power source, or correspond to a preferred acoustic mode of the resonator. When using a thermal acoustic power source, it is mainly the high resistance to the passage of the fluid imposed by the stack or regenerator units which determines its acoustic mode of operation by imposing the presence of speed knots (position where the speed is canceled) in the close vicinity of the regenerator units. Subsequently, the regenerator units will impose the presence of zones of high impedance. Thus the acoustic mode of the resonator is modified by the absence or presence of the second 13 and third 14 regenerator units (Figure 2). The presence of these two regenerator units generally has the effect of doubling the pulsation of the preferred acoustic wave. It is known that the optimal acoustic operating conditions of a regenerator unit correspond to an acoustic volume flow in advance with respect to the acoustic pressure at the end at “ambient” temperature of the regenerator unit, and in delay to its other end. FIG. 12A illustrates how the volume flow rate (first curve in solid and dotted line 40) and the sound pressure (second curve in continuous line 41) vary in a sound power transmission unit comprising a pipe element according to the second embodiment , ie having an equivalent length much less than λ / 4 acoustically. Figure 12B explains in a different and more detailed representation (Fresnel diagram) of the evolution of the phases and amplitudes of the pressure and the volume flow between the ends C and A ₂ of the acoustic power transmission unit and shows that the conditions ensuring optimal operation of each of the regenerator units are ensured. Between C and H the effect is capacitive in the acoustic sense, and the volume flow varies according to the first curve 40, and the acoustic pressure is generally preserved. A quantity of flow is taken from the first branch 42 to bring the acoustic volume flow to the inlet of the reduced diameter section 43 in advance with respect to the acoustic pressure. In the reduced diameter section 43, the effect is inductive in the acoustic sense and the acoustic pressure varies according to the second curve 41 and the flow rate is preserved. The acoustic flow being ahead of the acoustic pressure in H ^ this has the effect of increasing the amplitude of the acoustic pressure along the tube. The second bypass 44 will this time restore flow and make it possible to adjust the phase and the amplitude of the flow at A2. The favorable input conditions at the end of the second regenerator are satisfied, that is to say that the acoustic volume flow is ahead of the acoustic pressure in A2 and that the amplitude of the acoustic pressure in A2 is greater than that in C, in order to recover a sufficient dimensionless impedance. In addition, the invention has the other advantage of making it possible to adjust the phase of the volume flow rate at the end (A2) of the second regenerator regardless of its amplitude. In all cases, the use of a pipe element according to the second embodiment, ie a pipe element of equivalent length much less than λ / 4 acoustically, will be preferred between two regenerator units, provided that it can be used satisfactorily. An identified penalizing case can be, for example, the cascading of too many regenerator units. The present invention induces to correlate the position of the thermoacoustic units and of the transmission units which are interposed between the thermoacoustic units with the characteristic quantity Z of the sound field in the resonator. We speak of a zone of high dimensionless impedance when this is greater by an order of magnitude than 1 and of a zone of low dimensionless impedance otherwise. It is known that the stack and regenerator units must be placed in areas of high dimensionless impedance and values close to 5 are typically retained for a stack unit and 30 for a regenerator unit. A resonator section corresponding to a zero dimensionless impedance can be identified by local measurement of the sound pressure and determination of the section where it is canceled. A high dimensionless impedance zone corresponds to the resonator part where the value of the acoustic pressure amplitude in absolute value is maximum (Figure 11). Two main tube elements can also be connected not by a single tube of reduced diameter d but by a plurality of tubes of reduced diameter d ₀ or of different diameters dι, d _2l ... producing the same effect with regard to the transmission power (Figure 3). The change of section between the main tube and the tube or section of reduced diameter can be both discontinuous and continuous. In the first case, it can be a step, in the second, it can take the form of a cone. Figure 3 shows two main tube elements 1, 2 comprising respectively a stack unit 3 and a regenerator unit 4, or two regenerator units 3, 4. These thermoacoustic units 3, 4 are arranged in zones of adjacent high dimensionless impedance, which are separated by a zone of low dimensionless impedance. The two main tube elements 1, 2 are each connected at one of their ends by a plurality of tubes 5 of reduced diameter parallel to each other and to a branch 6, 7 comprising a cavity connected 8, 9 to a rectilinear conduit of circular section 10, 11. This embodiment proves to be advantageous when the acoustic powers to be transmitted are very high and it is necessary to reduce both the speeds in each of the tubes but also the diameters of each tube in order to avoid the significant wall thicknesses which are required by law with regard to the behavior of the device at the maximum operating pressure. In order to control and vary the at least part of the volume flow diverted from the main tube element to the derived cavity, the conduit leading to the cavity may include one or more resistive elements placed in series and acting positively on the phase of the flow at the inlet of the bypass.

These elements are chosen from the group comprising a diaphragm (Fig. 4), a compressible porous medium (Fig. 5) and a resistive valve (Fig. 6) or the like. Advantageously, the duct is controlled in temperature, whether by heating or cooling. For this, one can, for example, arrange the pipe in a thermostate bath whose temperature is adjusted either by heating said bath by an electric heating resistor or by cooling by means of an additional refrigeration unit. Electronic temperature control means adjust the temperature according to a set point (Fig. 7). Controlling the temperature of the duct advantageously allows non-intrusive adjustment of the acoustic characteristics. Figure 8 shows a bypass comprising a conduit 45 and a derived cavity 46. This cavity 46 includes an acoustically active element 47, for example, a stack unit or a loudspeaker mainly allowing active adjustment of the acoustic characteristics at the input of the diversion, but also to offset the losses due to dissipation, this mainly in the diversion. It is known that the association of a volume with a conduit such as a thin tube makes it possible to create an easily adjustable resonant cavity which can be acoustically qualified with a good approximation as a function of the volume of the cavity V and of section A and length I of the thin tube by the product: IV 2 ω, ArT where ω denotes the pulsation of the acoustic wave and T the temperature gas average expressed in Kelvin. For this quantity to be representative, the length of the thin conduit must be less than λ / 2π and the internal diameter di of this conduit must be such that di / δ _v "1 with δ _v the thickness of the viscous boundary layer and where δv = - / Pr ^{~ x} δk where P _r is the Prandtl number. In the case where the length of the reduced diameter section is IV 2 equivalent, acoustically, to λ / 2, ω is preferably greater than 5.

On the contrary when this length is much less than λ / 4 acoustically, it IV 2 is preferable to choose ω close to 2 but not equal or close to 1, ArT this to avoid dissipating all of the sound power of the tube main in the derivation. Figure 9 is a sectional view of a resonator having multiple leads in the same section according to an embodiment of the invention. Four main branches 49 are connected to the main tube element 48.

52 each comprising a straight line 53-56 and a derived cavity 57-60.

To avoid vibrations in the direction transverse to the axis of the main tube, a preferred embodiment is to arrange the branches 49-52 in pairs in directly opposite directions. The fields of application of thermoacoustic machines are varied and concentrated on refrigeration applications. The preferred fields of application of thermoacoustic refrigeration machines using heat as an energy source are, inter alia, the liquefaction of industrial or medical gases and industrial refrigeration.

Claims

CLAIMS 1. Power transmission unit for thermoacoustic systems comprising at least one stage, comprising: - at least two thermoacoustic units each comprising a regenerator or a stack and two heat exchangers, - an acoustic resonator comprising a tube and containing a fluid and in which establishes an acoustic field having zones of high dimensionless impedance and zones of low dimensionless impedance, - certain thermoacoustic units (3, 4, 12-16) being placed in zones of high dimensionless impedance, characterized in that: - each zone of high dimensionless impedance comprises at most one thermoacoustic unit, - two successive thermoacoustic units (3, 4, 12-16) being always separated by a zone of low dimensionless impedance, - the resonator comprises a section of reduced diameter (5 , 21 -23) between each of the pairs of successive thermoacoustic units, and in that each section narrowing (5, 21-23) is associated with at least one branch (6, 7) comprising a cavity (8, 9), said branch (6, 7) making it possible to divert at least part of the volume flow of the tube.

2. Power transmission unit according to claim 1, characterized in that each section narrowing is associated with two branches (6, 7), placed respectively at each end of said narrowing.

3. Power transmission unit according to claim 1 or 2, characterized in that the narrowing of the section is continuous.

4. Power transmission unit according to claim 3, characterized in that the narrowing of section takes the form of a cone.

5. Power transmission unit according to claim 1 or 2, characterized in that the narrowing of the section is discontinuous.

6. Power transmission unit according to claim 5, characterized in that the section narrowing takes the form of a step.

7. Power transmission unit according to any one of claims 1 to 6, characterized in that each branch (6, 7) comprises a conduit (10, 11) connecting the cavity (8, 9) to the tube.

8. Power transmission unit according to claim 7, characterized in that each branch additionally comprises thermal regulation means (6, 7) making it possible to control the flow rate in the branch.

9. Power transmission unit according to claim 7 or 8, characterized in that resistive systems are associated with at least one of the conduits.

10. Power transmission unit according to any one of claims 1 to 9, characterized in that it comprises at least one acoustically active element (47) allowing the adaptation of the operating conditions of the thermoacoustic units (3, 4, 12-16).

11. Power transmission unit according to claim 10, characterized in that said acoustically active element (47) is a stack unit placed in the derived cavity.

12. Power transmission unit according to claim 10, characterized in that said acoustically active element (47) is a loudspeaker placed in the derived cavity.